Ab initio Lattice Dynamics Double Perovskite

摘要

Pb_2MgTeO_6 is one of the few perovskites presenting an incommensurate phase at low temperature. We report calculations of the lattice dynamical properties (phonon band structure) for the cubic, (high-T) and the average rhombohedral, (low-T) structures of Pb_2MgTeO_6. The cubic phase presents two unstable phonons at Γ. The first has a T_(1u) character and is restricted to a small neighborhood near Γ. The second has a T _(1g) character in Γ. Its lowest branch generates, in the reciprocal space, tube-like unstable regions along ΓX. Its displacement pattern, in the real space, corresponds essentially to rotations of the TeO_6 octahedra. The rhombohedral average structure presents no instability in Γ. However, an instability is obtained by interpolation along the 100 direction, around the q= < δ00 > point, with δ close to 0.1, in disagreement with a published interpretation of experimental data, claiming q= <δδδ >. Reexamination of the published electron diffraction spectra shows that the correct value of the modulation wavevector is q= < δ00 >.